All linear amplifiers have a finite overload limit. In satellite communications systems this restriction is of great importance because of the limited power available on the satellite and size and weight limitations of the satellite equipment.
FIG. 1 is a curve of a typical linear amplifier used in satellite transponders. Many transponders use travelling wave tube amplifiers (TWTA) and much work has been done to replace such amplifiers with solid state power amplifiers (SSPA) because of their greater linearity and also higher reliability. But even the best of linear amplifiers create distortion and the transponder's linear power amplifier is the weakest link in the overall satellite communications system. Such amplifiers produce intermodulation distortion that causes a noise like product when a large number of signals are frequency multiplexed. For an excellent discussion of the envelope and phase linearities satellite systems see J. J. Spilker, Jr., "Digital Communications by Satellite", Prentice-Hall, 1977, Englewood Cliffs, N.J., especially Chapter 9.
(This problem can be somewhat alleviated by the use of the envelope elimination and restoration (EER) system as disclosed in applicant's patents, for example U.S. Pat. No. 2,666,133 and described in a number of papers, for example L. R. Kahn, "Comparison of Linear Single-Sideband Transmitters with Envelope Elimination and Restoration Single-Sideband Transmitters", Pro. IRE, December 1956, page 1706-1712. But, of course, even the EER system is not distortion free.)
The instant invention is described in terms an overall system for delivering a large number (300) of high fidelity stereo channels utilizing only 18 mHz of a C-Band satellite system. Applicant's co-pending application Ser. No. 08/088,123 filed on Jul. 8, 1993 discloses such a system including a novel encryption system and that application is incorporated by reference herein.
However, the invention may be applied to all types of modulation and to various forms of information transmitted over the satellite channels and is not limited to ground-to-ground satellite systems but also can be used to increase the range and performance of satellite-to-satellite communications systems and to terrestrial systems where unattended repeaters such as used with submerged cable systems.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with accompanying drawings, and its scope will be pointed out in the appended claims.